In the Body, What Is the Role of Thyroxine?

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  • Written By: Liz Thomas
  • Edited By: Shereen Skola
  • Last Modified Date: 10 September 2019
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Thyroxine is one of the main hormones secreted from the thyroid follicular cells. Within the body, thyroxine affects the body’s metabolism and has an effect on protein synthesis. It is also important for human growth and development, as it regulates bone growth and aids in the maturation of neurons. This hormone also increases the catecholamine effect, the body’s sensitivity to adrenaline. There are health issues linked to low and high levels of this hormone in the body.

Thyroxine is also known as T4, as the molecular structure consists of thyronine molecule with four iodine molecules. Synthesis occurs in the thyroid gland. The hormone is released into the blood through the thyroid follicular cells. Approximately 80 percent of all T4 is produced in the follicular cells.

In the body, T4 is transported to cells via the blood. Only free or unbound hormones are passed into cells and considered active. Up to 99 percent of thyroxine is actually bound to proteins in the blood. An individual’s concentration of free T4 in the blood is extremely important in medical investigation and diagnosis.


This particular hormone influences metabolism by regulating how much oxygen the cells use, and by creating body heat. Within the liver, T4 directly affects cellular respiration, the process of transforming glucose and oxygen into energy and carbon dioxide. The presence of thyroxine promotes the first step of metabolism, going from glucose to pyruvate. Within the stomach and intestines, T4 promotes muscle contraction and digestive juice secretion.

As children age, T4 is responsible for regulation of growth and development. The hormone coordinates bone growth with age, as well as development of cardiac and skeletal muscles. In the skin, T4 is responsible for hair growth. As children mature, the presence of the hormone results in breast milk production and the ability to conceive. During developmental years, T4 also is responsible for normal development of nerves and neurons.

The catecholamine effect refers to the body’s response to stress, invoking the flight or fight response in the body. This response is due to the presence of adrenaline and noradrenaline in the body. The presence of thyroxine increases the body’s sensitivity to catecholamine compounds.

An overactive or underactive thyroid can result in issues with T4 levels in the body. Too much T4, due to hyperthyroidism, results in nervousness, weight loss, and other potentially serious health issues. An under-active thyroid can result in a number of other health conditions, including developmental problems and weight gain. Supplements may be used to increase the level of thyroxine in the body. Overactive thyroids are sometimes destroyed or removed and hormone therapy used to provide the necessary hormones for proper body function.


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How does the body respond to high levels of thyroxine?

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